Over the past several decades ultra-sensitive magnetometers have found a wide range of applications, from condensed matter experiments (Tsuei, C. C. et al., Phys. Rev. Lett. 85, 182-185 (2000)) and gravitational wave detection (Harry, G. M. et al., Appl. Phys. Lett. 76, 1446-1448 (2000)), to detection of nuclear magnetic resonance (NMR) signals (Greenberg, Ya. S., Rev. Mod. Phys. 70, 175-222 (1998); McDermott, R. et al., Science 295, 2247-2249 (2002)), studies of paleomagnetism (Kirschvink, J. L. et al., Science 275, 1629-1633 (1997)), non-destructive testing (Tralshawala, N. et al., Appl. Phys. Lett. 71, 1573-1575 (1997)), and ordinance detection (Clem, T. R., Nav. Eng. J. 110, 139-149 (1998)). For the last 30 years superconducting quantum interference devices (SQUIDs) operating at 4K have been unchallenged as ultra-high-sensitivity magnetic field detectors (SQUID Sensors: Fundamentals, Fabrication and Applications, Ed. Weinstock, H., Kluwer Academic (1996)) with a sensitivity reaching down to 1 fT/Hz1/2 (where fT designates femtotesla, or 10−15 tesla).
Detection of radio-frequency (RF) fields in the kilohertz to gigahertz frequency range finds numerous applications, from radio communication to detection of NMR and NQR signals to fundamental physics measurements, such as axion searches (Bradley, R. et al., Rev. Mod. Phys. 75, 777 (2003)). These applications involve the detection of extremely small fields and thus require the highest attainable sensitivity. While RF fields are usually detected with inductive pick-up coils, several other methods, such as SQUID magnetometers (Black, R. C. et al., Appl. Phys, Lett. 66, 1267 (1995); Seton, H. C. et al., IEEE Trans. Appl. Supercond. 7, 3213 (1997)) and Rydberg atoms (Ogawa, I. et al., Ploys. Rev. D 53, R1740 (1996)) have been used for this purpose. Alkali-metal atomic magnetometers, which measure the response of optically-pumped, polarized atoms to a magnetic field, have achieved very high magnetic field sensitivity at frequencies below 100 Hz (Kominjs, K. et al., Nature 422, 596 (2003)), and provide an important alternative to SQUID instruments.
Atomic magnetometers are based on detection of Larmor spin precession of optically pumped atoms. Alkali metal magnetometers have approached sensitivity levels similar to SQUID instruments when using large measurement volumes (Aleksandrov, E. B. et al., Optics and Spectr. 78, 292-298 (1995); Budker, D. et al., Phys. Rev. A 62, 043403 (2000)), but have much lower sensitivity in more compact designs suitable for magnetic imaging applications (Affolderbach, C. et al., Appl Phys B 75, 605-612 (2002)).
Spin exchange in alkali metal vapors has been discussed. Happer W. et al. Phys. Rev. Lett. 31, 273 (1973) and Happer W. et al. Phys. Rev. A 16 1877 (1977) report experimental and theoretical aspects of observing magnetic resonance in high density alkali metal vapors in the presence of a buffer gas. U.S. Pat. No. 4,005,355 to Happer et al. discloses a high-density alkali vapor optically pumped to produce a narrow magnetic resonance line with a frequency proportional to a magnetic field.
Bison et al. (a) (Appl. Phys. B. 76, 325 (2003) and Bison et al. (b) (Opt. Expr. 11, 908 (2003)) disclose an optically pumped cesium atom magnetometer for use in dynamic cardiac magnetic imaging. Observed magnetic noise levels in Bison et al. (b) appear to be on the order of 1000 fT/Hz1/2.
Upschulte et al. (U.S. Pat. No. 6,472,869) discloses a diode laser-pumped alkali magnetometer. In Upschulte et al., response radiation includes photons that indicate one unit of angular momentum indicative of the torque due to the magnetic field, and a photodiode and scope that act as a means for measuring the response radiation. Upschulte et al. disclose a projected sensitivity of less than 6 pT/Hz1/2 (pT=picotesla or 10−12 tesla).
In view of the disadvantages of relatively poor sensitivity, and drawbacks such as large bulk and use of cryogenic systems summarized above, there remains a need for a magnetometer that can operate in the absence of expensive liquid helium dewars needed to maintain superconducting conditions, and also to avoid the need for other liquefied gas dewars used with higher temperature superconducting devices. In addition there remains a need for the development of advantageous atomic magnetometers with high sensitivity. There further is a need for a compact magnetometer that is relatively inexpensive to assemble and operate.